There are several pins with multiple features. Actually pin 4 is the only one which hasn't got special features (yet!). For a complete list, see http://www.netduino....duino/specs.htm
SPI is also explained a bit at http://wiki.netduino.com/SPI.ashx
Thanks Stefan - now it makes sense. I was reading "SPI_Devices.SPI1" as a single pin (hadn't decided if it meant 11 or 13...), but now I get that it is the 3-pin "set" of 11, 12, 13 - thus declaring pin 12 as "used". I imagine that using UART1 or UART2 or I2C devices will have a similar behavior.